This article provide you help while developing application for different environments. Like if you want to separate config file for staging and another for live and another for QA. You can use Transform Web.config method in visual studio to accomplish multiple configuration file in single project.

Transform Web.config

This feature does however require you to have the VS2012 Update 2 or the Azure SDK 2.0 for VS2010.
In Visual Studio, navigate to the Solution Configuration drop down list and choose "Configuration Manager".

Next, look for the Active solution configuration drop down and select "New".

Give your configuration a name - I am going to call mine "Staging". I am also going to copy the settings from another transform.

That's it! Next time you deploy simply choose the appropriate solution configuration and your settings will be applied.

USING THE PUBXML FILE

You can also achieve the same result by using a publish profile file (.pubxml). Start by navigating to your web project in the Solution Explorer and choosing "Add Config Transform". This will add a new config transform for that publish profile only.

Once complete, you should notice that the appropriate Web.Config transform has been created.

When you deploy your next project, simply select the solution configuration that you need from the Solution Configuration drop down and the settings in your Web.Config file will be applied.